Heretofore, various proposals have been made with reference to a method for producing synthetic pulp from synthetic fibers.
As methods using a polymer of the acrylic series, known methods include making the polymer a porous gel fiber and then beating the said fiber, treating the polymer with a liquid agent such as strong mineral acid, a swelling agent, a determining agent, and promoting fibrillation by polymer blending. Methods using a polymer of the olefin series, include splitting a film to bring about fibrillation. However, because by such means the resulting pulp does not acquire hydrophilic properties, satisfactory dispersing properties in water at the time of wet paper making, and satisfactory adhesion of fibrils after the sheet is formed, have not been obtained.
On the other hand, from the viewpoint of performance of the paper, synthetic pulps have not been known which are capable of producing a paper having a high degree of opacity and having high wet dimensional stability. As materials for improving such characteristics of wood pulp, various inorganic fillers or powdered synthetic polymers have been used. However, they have no paperforming capability and can be used only as additives, and do not achieve the objects of the present invention.
Specifically, Japanese Pat. application Publication No. 10655/1964 discloses fibrillating synthetic fibers in a swelling agent to obtain so-called hooked fibers having short, fine hooks. In such hooked fibers, however, the so-called hooks necessary for intertwining are short and are not self-adhesive, and therefore intertwining of fibers is not sufficient for the purpose and the paper that is obtained is non-uniform.
It is disclosed in Japanese Pat. application Publication No. 20757/1961 that gel-like, non-collapsed, wet spun acrylic fibers tend to become fibrillated.
However, fibrillation is carried out, as reported in said publication using gel-like, non-collapsed fibers. In order to convert fibrillated fibers into a sheet-like form, it is preferable that they should have hydrophilic properties, and self-adhesion, as well.
If fibrillated fibers should fail to exhibit selfadhesion, a sheet-like material having high tensile strength cannot be obtained, a special adhesive is required in order to obtain high tensile strength. A sheet as obtained in the abovementioned publication is, as described in the specification, very low in tensile strength, which clearly possesses the drawback that the fibrillated material fails to exhibit the property of self-adhesion.
Japanese Pat. application Publication No. 11851/1960 discloses a synthetic pulp fibril having feeler-like protrusions which are said to be capable of intertwining. As distinguished from the product obtained by ordinary spinning processes, and because this fibril essentially lacks any so-called stalk fibers, it has the drawback, when synthetic paper is made therefrom, that the paper product has poor physical properties, especially tenacity. Accordingly, this fibril, together with other fibrous materials, is effectively utilized as an adhesive but it has the disadvantage that good paper cannot be obtained from this fibril alone.
We, the present inventors have previously proposed, as referred to in our U.S. applications Ser. Nos. 180,875 and 435,453, fibers for paper and synthetic pulp consisting of (A) 15-90 percent by weight of a fiber-forming hydrophobic polymer and (B) 10-85 percent by weight of a copolymer in which a hydrophilic portion and a hydrophobic portion are bonded chemically, in which the hydrophilic portion is dispersed and oriented in the direction of the fiber axis.
However, we have conducted strenuous studies with a view to producing a fiber which is capable of yielding a pulp having even more excellent opacity and wet dimensional stability in respect of paper performance, high level dispersing properties in water and fibril adhering strength after forming the sheet, and we have now created a fiber which has the outstanding composition and structure of the present invention as a result.
An object of the present invention is to provide a method of making a fiber for a synthetic paper which is capable of yielding paper having high opacity, degree of whiteness and wet dimensional stability not attainable by paper made of conventional wood pulp.
Another object of the present invention is to provide a method of making a fiber for a synthetic paper imparted with performances capable of producing pulp and making paper by exactly the method normally used for wood pulp. This is done by imparting hydrophilic properties to pulp -- a point which has been little considered in the case of conventional synthetic pulp, improving the dispersion properties in water when made into an aqueous slurry, and imparting adhesion strength between the fibrils.
Still another object of the present invention is to provide a method for making a synthetic fiber which is capable of obtaining a beaten fibril (pulp) having excellent intertwinement and adhesion among the fibrils, which can be made into paper in a wet system, using ordinary beating means of the type normally used with natural pulp, and capable of using the resulting beaten fibril in making paper consisting of 100 percent of said beaten fibrils, or making paper consisting of beaten fibril and natural pulp in desired ratios.
These and other objects will further become apparent in the following specification, and in the claims.